When people spend a ton of time making custom texture sets and sell them for really high tc, can't people just copy the textures once they play with the person who bought the texture

It's against rules, if you aren't the owner of a certain texture, you can't wear it.

Technically. They can. They could give it to any person they like. But there are severe repercussions. Only one person is allowed to own the texture, and if he gives it to another person, then incidentally it is now that persons texture.
-This does not apply to free textures.

and if the player takes the textures from the persons activated inventory the picture wont be good quality

Any kind of texture theft, including recoloring or even using the head as a template to draw a different one are not allowed, and the person who'd wear such head would have to immediately take it off or be banned